Split power continuously variable transmissions (CVTs) and hydrostatic transmissions (hystats) have certain advantages over more conventional transmission types used on earth moving and agriculture machines. In the split power CVT machine, in addition to an engine directly driving a mechanical transmission system, a hydraulic motor, typically of fixed displacement and variable hydraulic pump, is used as a variator to provide continuously variable torque and speed to the drive train. In a Hydrostatic machine, or “hydrostat,” the hydraulic motor is used directly without the additional mechanical drive from the engine.
In either case, with the continuously variable speed of the transmission provided in these arrangements, higher transmission efficiency can be obtained relative to a torque converter transmission. In addition, the engine can be regulated close to its optimum operating point (e.g., peak torque, and/or peak rpm, and/or minimum emissions per power unit) so that better performance can be expected for machines equipped with hydrostatic or parallel path type transmissions as described above. In a machine equipped with such a transmission, the machine speed or drawbar power can be controlled by controlling the hydraulic motor (e.g., controlling swash plate position of hydraulic pump, and/or controlling hydraulic motor output torque). In short, split power CVT machines and hydrostatic drive machines typically function in a stable and beneficial manner relative to other drive types.
However, such transmissions have certain inherent limitations, most notably with respect to the size of the hydraulic pumps and motors. In particular, it has been difficult to date to scale up either system type to account for much larger machine sizes. For example, larger displacement hydraulic pumps and motors inherently possess much more limited operating speed capabilities than smaller pumps and motors. Moreover, large-sized piston hydraulic displacement machines tend to be less efficient than their smaller counterparts. In addition, larger-sized actuators will be required for controlling larger pumps and motors, thus requiring larger control valves to handle the higher control flow requirements, which further complicates the system design.
While the disclosed principles herein are directed at least in part to overcoming one or more disadvantages, noted or otherwise, it will be appreciated that the innovations herein are defined by the attached claims without to regard to whether and to what extent the specifically claimed embodiments overcome one or more of the noted problems in the existing technology. Moreover, it will be appreciated that any discussion herein of any reference or publication is merely intended as an invitation to study the indicated reference itself, and is not intended to replace or supplement the actual reference. To the extent that the discussion of any reference herein is inconsistent with that reference, it will be appreciated that the reference itself is conclusive as to its teachings.